Adhesives are often used to bond substrates together. In the area of industrial adhesives, hot melt adhesives are commonly used to bond together a wide variety of articles including disposable absorbent articles comprising non-woven substrates e.g. diapers, training paints, surgical garments, swim wear, absorbent underpants, adult incontinence products, sanitary napkins and medical dressings (e.g. wound care products).
There can be multiple hot melt adhesives used in the manufacture of a disposable absorbent article. For example, in the manufacture of a disposable diaper, hot melt adhesives are used for construction (e.g. bonding the backsheet to the nonwoven and optionally the absorbent pad), elastic attachment (e.g. bonding the elastic material to the backsheet in for example the leg or waist area), and for the core stabilization (e.g. applying an adhesive to the absorbent core to increase the strength of the core).
Hot melt adhesives can also be used to form elastic composites that are useful in disposable articles. Currently, elastic composites are often formed in a 5-layer configuration including the following layers: nonwoven, hot melt adhesive, elastic material, hot melt adhesive, nonwoven. The hot melt adhesive bonds the non adhesive elastic to the nonwoven to form a composite.
Alternately, a hot melt adhesive with elastic properties can replace both the elastic material and the adhesive layers to form a simplified 3-layer elastic composite that can impart stretch to various portions of the disposable article.
Styrene block copolymers have been used as a polymer in such compositions.
Melt flow rate (MFR) is inversely related to the viscosity of a polymer. A high MFR means that a polymer has a low viscosity. Commercially available styrene block copolymers (SBC) work well in adhesives, but are not commonly commercially available with a high melt flow rate (i.e. >than 15 g/10 min (190° C., 2.16 kgs)). The few grades that are available, generally achieve the high melt flow rate with a high di-block content, which lowers the mechanical properties of the polymer, and those of the adhesive as well. This makes it difficult to formulate a low viscosity hot melt adhesive with good elastic recovery and high peel adhesion.
It would be desirable to be able to formulate a low viscosity hot melt adhesive based on SBC, with good elastic recovery so as to be useful in constructing a 3-layer elastic composite.